Ohmic electrode and method for forming it

Abstract: An ohmic electorde of the present invention comprises a contact electrode layer formed on a p-type diamond semiconductor layer formed on a substrate so as to be in ohmic contact with the p-type diamond semiconductor layer and to have low contact resistance and high heat resistance, and a lead electrode layer formed on the contact electrode layer so as to have low lead wire resistance and high heat resistance. Specifically, the contact electrode layer is made of either a carbide of at least one metal selected from a metal group comprising Ti, Zr, and Hf, or a carbide of an alloy containing at least one metal selected from the metal group. Since the carbide of the metal or alloy forming the contact electrode layer is stabler in respect of energy because of reduced formation enthalpy than the metal or alloy itself, it is very unlikely to diffuse. Therefore, little metal or alloy forming the contact electrode layer precipitates on the surface of the lead electrode layer formed on the contact electrode layer, thus improving the device performance, based on the reduced lead wire resistance.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an ohmic electrode with high heat resistance and low contact resistance, formed on a p-type diamond semiconductor and a method for forming it. Related Background Art Diamond has a large bandgap of about 5.5 eV and thus has no temperature range corresponding to the intrinsic region below 1400 °C. It is, therefore, excellent in heat resistance. Also, diamond has the thermal conductivity of 20 W/cm·K, which is ten times larger than that of silicon. Thus, it is also excellent in heat radiation. Because of such properties, semiconductor devices made of diamond can be operated at high temperature and can be improved in the degree of integration of circuits. These days, p-type diamond semiconductors formed by doping diamond with boron are expected to be applied to light-emitting devices to emit light in the range of various wavelengths from ultraviolet light to visible light, environmentally resistant semiconductor devices required to have high heat resistance and high thermal conductivity, and etc. In order to produce such semiconductor devices, ohmic electrodes formed on the p-type diamond semiconductor need be provided with high heat resistance and low contact resistance. A conventional ohmic electrode is formed by successively building a contact electrode layer of Ti, a diffusion barrier layer of Mo, and a lead electrode layer of Au up on a p-type diamond semiconductor. Ti forming the contact electrode layer is a metal having a relatively high melting point, but has a resistivity of higher than materials such as Au, Al, etc. Thus, Au is layered as a lead electrode layer above the contact electrode layer so as to reduce lead wire resistance in order to improve device performance. However, when the lamination structure of Ti and Au is subjected to heat treatment at a temperature of about 300 °C, Ti forming the contact electrode layer readily diffuses into Au forming the lead electrode layer to precipitate over the surface of the lead electrode layer, thus increasing the lead wire resistance. Even in use at room temperature current supply to such a semiconductor device causes Joule heat generated to induce mutual diffusion of Ti and Au, which results in precipitation of Ti over the surface of the lead electrode layer. To prevent the diffusion of Ti, Mo is interposed as a diffusion barrier layer between the contact electrode layer and the lead electrode layer. The above prior art technology is described in detail in Japanese Laid-open Patent Applications No. 1-246867 and No. 2-260470. In the above conventional ohmic electrode the thickness of Ti forming the contact electrode layer is, however, relatively large, i.e., about 300 to 500 nm. Then, the interposition of Mo as the diffusion barrier layer between the contact electrode layer and the lead electrode layer cannot fully prevent Ti from diffusing from the contact electrode layer toward the lead electrode layer. As a result, Ti precipitates over the surface of the lead electrode layer of Au, which raised a problem of degrading the device performance because of an increase in lead wire resistance. US-A-5055424 discloses an ohmic electrode formed by heating, at a high temperature, a carbide forming metal deposited over a semiconductor diamond and made of a metal selected from a group including Ti, Zr, and Hf, and a corrosion resistant metal selected from a group including Au. However, this reference does not disclose that the whole of a layer region between the semiconductor diamond ant the corrosion resistant metal is constituted by the carbide of the specific metal or alloy and that the semiconducting diamond has a P-type. Japanese Journal of Applied Physics vol 28, no 5.I, May 1989, pages 758-762 discloses an ohmic electrode comprising a Ti-layer deposited at 400°C on a p-type semiconductor diamond substrate. However, this reference does not disclose that the whole region of the Ti-layer is constituted by the carbide of Ti and that the ohmic electrode comprises an Au-layer formed on the Ti-layer. EP-A-0390209 disclose an ohmic Ti electrode formed on a boron doped semiconductor diamond. The present invention has been accomplished to solve the above problem. It is, therefore, an object of the present invention to provide an ohmic electrode formed on a p-type diamond semiconductor, which can improve the device performance, based on reduced lead wire resistance, and a method for forming the ohmic electrode. SUMMARY OF THE INVENTION Achieving the above object, an ohmic electrode of the present invention comprises a contact electrode layer formed on a p-type diamond semiconductor layer formed on a substrate so as to be in ohmic contact with the p-type diamond semiconductor layer and to have low contact resistance and high heat resistance, and a lead electrode layer made of Au formed on the contact electrode layer so as to have low lead wire resistance and high heat resistance, in which the whole contact electrode layer is made of either a carbide of at least one metal selected from a metal group comprising Ti, Zr, and Hf, or a carbide of an alloy containing at least one metal selected from said metal group. Achieving the above object, a method for forming an ohmic electrode of the present invention comprises a first step of vapor-depositing either at least one metal selected from a metal group comprising Ti, Zr, and Hf or a carbide thereof, or an alloy containing at least one metal selected from the said metal group or a carbide thereof on a p-type diamond semiconductor layer formed on a substrate to form a contact electrode layer being in ohmic contact with the p-type diamond semiconductor layer and having low contact resistance and high heat resistance, and a second step of forming by vapor deposition a lead electrode layer made of Au on the contact electrode layer formed in the first step, in which the second step is sequentially performed after said first step and said first step comprises a heat treatment of the contact electrode layer at a temperature of not lower than 300 °C. In the ohmic electrode of the present invention, the whole contact electrode layer made of either the carbide of at least one metal selected from the metal group comprising Ti, Zr, and Hf, or the carbide of an alloy containing at least one metal selected from said metal group is formed on the p-type diamond semiconductor layer formed on the substrate, as being in ohmic contact therewith. The inventors of the present application have verified that the contact resistance was low between the contact electrode layer and the p-type diamond semiconductor layer. Further, the inventors of the present application presumed that the diffusion was very unlikely to occur, because the carbide of the metal or the alloy forming the contact electrode layer was stabler in respect of energy because of reduced formation enthalpy than the metal or alloy itself. According to the method for forming the ohmic electrode, of the present invention, the first step comprises the heat treatment to heat the substrate at a temperature of not lower than 300 °C in forming the contact electrode layer by vapor-depositing either at least one metal selected from the metal group comprising Ti, Zr, and Hf or a carbide thereof, or an alloy containing at least one metal selected from said metal group or a carbide thereof. In another embodiment, such a contact electrode layer is first formed and thereafter the heat treatment is carried out by heat the contact electrode layer in an atmosphere having a temperature of not lower than 300 °C. The heating causes the metal or alloy forming the contact electrode layer to thermally diffuse into the p-type diamond semiconductor layer, and vice versa, reacting with diamond to form its carbide. Comparing with the method for directly forming the carbide of the metal or alloy forming the contact electrode layer on the p-type diamond semiconductor layer, the contact resistance to the p-type diamond semiconductor layer is reduced, because a superior interface with less defects is formed between the p-type diamond semiconductor layer and the contact electrode layer. The metal or alloy forming the contact electrode layer becomes stabilized in respect of energy by the formation of carbide, so that the metal or alloy is unlikely to diffuse. Since the carbide of the metal or alloy forming the contact electrode layer is stabler up to near the melting point higher than that of the metal or alloy itself, it is excellent in heat resistance. Accordingly, the ohmic contact with high heat resistance and low contact resistance can be formed with excellent reproducibility on the p-type diamond semiconductor layer. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a cross-sectional view to show the structure of a first embodiment associated with an ohmic electrode of the present invention; DESCRIPTION OF THE PREFERRED EMBODIMENTS The structure and operation of embodiments according to the present invention will be described with reference to Fig. 1. In the description with the drawings identical elements will be denoted by same reference numerals and redundant description will be omitted. It should be noted that the ratio of the dimensions in the drawing does not always coincide with that in the description. In the following description, a IVA-group metal or a IVA-group alloy means at least one metal selected from a metal group consisting of Ti, Zr, and Hf, or an alloy containing at least one metal selected from the said metal group, respectively. Fig. 1 shows the structure of the first embodiment associated with an ohmic electrode of the present invention. A contact electrode layer 30 and a lead electrode layer 50 are successively layered on a p-type diamond layer 20 formed on a substrate 10. The contact electrode layer 30 is made of a carbide of a IVA-group metal or alloy with high heat resistance, which is in ohmic contact with the p-type diamond layer 20. The lead electrode layer 50 is made mainly of a substance such as Au. The operation of the first embodiment is next described. Since the carbide of the metal or alloy forming the whole contact electrode layer 30 is much stabler in energy because of the reduced formation enthalpy than the metal or alloy itself, it is very unlikely to diffuse. Next described is a method for forming the above ohmic electrode in the first embodiment. First, a IVA-group metal or alloy, or a carbide thereof is vapor-deposited on the p-type diamond layer 20 formed on the substrate 10 to form the contact electrode layer 30. Then Au is vapor-deposited on the contact electrode layer 30 to form the lead electrode layer 50. The vapor deposition method may be selected from various techniques including the ion plating method, the arc ion plating method, the sputter deposition method, the vacuum vapor deposition method, and the electron beam evaporation method. Also, the contact electrode layer 30 may be formed in such a manner that the substrate is first set at a temperature of not lower than 300 °C by heating an electrode portion of the substrate 10 and then the IVA-group metal or alloy is vapor-deposited on the p-type diamond layer 20. Further, the heat treatment of the contact electrode layer 30 may be such that after the IVA-group metal or alloy has been vapor-deposited on the p-type diamond layer 20, the resultant is annealed in an atmosphere of vacuum or Ar gas as heated at a temperature of not lower than 300 °C. Further, after the formation of the contact electrode layer 30 and before the formation of the lead electrode layer 50, the contact electrode layer 30 may be washed with an acid solution such as hydrofluoric acid (HF) or hydrofluoric acid/nitric acid (HF/HNO 3 ). Moreover, if the thickness of the contact electrode layer 30 is not more than 200 nm, a heat treatment may be conducted in forming the lead electrode layer 50 in the same manner as in the formation of the contact electrode layer 30. Next described are the operational effects of the method for forming the ohmic electrode in the first embodiment. Since the carbide of the IVA-group metal or alloy has the reduced formation enthalpy as compared with the metal or alloy itself, the vapor deposition of the carbide of the IVA-group metal or alloy on the p-type diamond layer 20 can keep the metal or alloy forming the contact electrode layer 30 stabler in energy whereby the metal or alloy is very unlikely to diffuse. When the IVA-group metal or alloy is vapor-deposited on the p-type diamond layer 20 and then the heat treatment is conducted at a temperature of not lower than 300 °C, the metal or alloy forming the contact electrode layer 30 thermally diffuses toward the p-type diamond layer 20 and vice versa, reacting with diamond to form the carbide thereof. Comparing with the case where the carbide of the metal or alloy forming the contact electrode layer 30 is directly formed on the p-type diamond layer 20, an excellent interface with less defects is formed between the p-type diamond layer 20 and the contact electrode layer 30, thus reducing the contact resistance to the p-type diamond layer 20. As described, the metal or alloy forming the contact electrode layer 30 is stabilized in respect of energy by the formation of the carbide thereof, which hardly diffuses. Also, the carbide of the metal or alloy forming the contact electrode layer 30 is stable up to near the melting point higher than that of the metal or alloy itself, and, therefore, it is excellent in heat resistance. By washing the contact electrode layer 30 with the acid solution after the formation of the contact electrode layer 30 and before the formation of the lead electrode layer 50, the metal or alloy remaining without forming the carbide is removed from the contact electrode layer 30. Further, adhesion is improved between the layered contact electrode layer 30 and lead electrode layer 50 by forming the lead electrode layer 50 as heated similarly as the contact electrode layer 30 in case of the thickness of the contact electrode layer 30 being not more than 200 nm. The following describes experiments as to the first embodiment. First, Table I shows formation conditions and performance evaluation results for the above first embodiment. Samples No. Al through A7 were prepared in such a manner that while the substrate 10 and the atmosphere were held at the room temperature, a carbide of a IVA-group metal or alloy was vapor-deposited as the contact electrode layer 30, and, subsequently, the substance such as Au was vapor-deposited in the thickness of about 200 nm as the lead electrode layer 50. Also, samples No. A8 through A12 were prepared as follows. The substrate 10 was first heated up to a temperature of not lower than 300 °C and then a IVA-group metal or alloy was vapor-deposited as the contact electrode layer 30. Next, the temperature of the substrate 10 was reduced down to the room temperature and then the substance such as Au was vapor-deposited in the thickness of about 200 nm as the lead electrode layer 50. Further, samples No. A13 through A15 were prepared as follows. The substrate 10 was first heated up to a temperature of not lower than 300 °C and then a carbide of a IVA-group metal or alloy was vapor-deposited as the contact electrode layer 30. Next, the temperature of the substrate 10 was reduced down to the room temperature and the substance such as Au was vapor-deposited in the thickness of about 200 nm as the lead electrode layer 50. Additionally, samples No. A16 through A19 were prepared as follows. While the temperature of the substrate 10 and the atmosphere was held at the room temperature, a IVA-group metal or alloy was vapor-deposited as the contact electrode layer 30, and thereafter the resultant was subjected to an annealing treatment to heat it at a temperature of not lower than 300 °C in vacuum of below 1333,2 Pa (10 Torr), preferably below 666,6 Pa (5 Torr) for thirty minutes. Next, the temperature of the atmosphere was reduced down to the room temperature and then the substance such as Au was vapor-deposited in the thickness of about 200 nm as the lead electrode layer 50. Moreover, samples No. A20 through A22 were prepared as follows. While the temperature of the substrate 10 and the atmosphere was held at the room temperature, a IVA-group metal or alloy was vapor-deposited as the contact electrode layer 30, and thereafter the resultant was subjected to an annealing treatment to heat it at a temperature of not lower than 300 °C in an atmosphere of Ar gas under the pressure of below 133,32 Pa (1 Torr) for thirty minutes. Next, the temperature of the atmosphere was reduced down to the room temperature and then the substance such as Au was vapor-deposited in the thickness of about 200 nm as the lead electrode layer 50. Further, samples No. A23 through A25 were prepared as follows. While the temperature of the substrate 10 and the atmosphere was held at the room temperature, a IVA-group metal or alloy was vapor-deposited as the contact electrode layer 30, and thereafter the resultant was subjected to an annealing treatment to heat it at a temperature of not lower than 300 °C in vacuum of below 1333,2 Pa (10 Torr), preferably below 666,6 Pa (5 Torr) for thirty minutes. Next, the contact electrode layer 30 was washed with an acid solution such as hydrofluoric acid or hydrofluoric acid/nitric acid to remove the metal or alloy remaining without forming the carbide. Subsequently, the temperature of the atmosphere was reduced down to the room temperature and then the substance such as Au was vapor-deposited in the thickness of about 200 nm as the lead electrode layer 50. The thus formed samples No. Al through A25 were subjected to a heat treatment at a temperature of 500 °C in vacuum of below 133,32·10 -5 Pa (10 -5 ) Torr for thirty minutes. The resistivity of the lead wire resistance was measured by the four-probe method before and after the treatment to evaluate the heat resistance, based on a change in the resistivity. The results of the measurement demonstrated that the lead wire resistance of each sample was nearly equal to a resistance value of Au normally formed in bulk before and after the heat treatment and that a change rate of the wire resistance of each sample was small. It was also observed that the surface of the lead electrode layer 30 had a color after the heat treatment, nearly identical with its original color. It is thus understood that high heat resistance was well achieved. In another experiment, samples No. Al through A25 were subjected to a heat treatment at a temperature of 500 °C in vacuum of below 133,32·10 -5 Pa (10 -5 Torr) for thirty minutes. The contact resistance was measured by the TLM method before and after this treatment to evaluate the ohmic property to the p-type diamond layer 20, based on a change in the contact resistance. The results of the measurement demonstrated that the contact resistance of each sample was kept nearly unchanged before and after the heat treatment as to be a value below 1 × 10 -3 Ω·cm 2 . It is thus understood that excellent ohmic property was achieved. Next, Table II shows formation conditions and performance evaluation results of comparative examples to the above first embodiment. Samples No. B1 through B6 were prepared as follows. While the temperature of the substrate 10 and the atmosphere was held at the room temperature or at a temperature of lower than 300 °C, a IVA-group metal or alloy was vapor-deposited as the contact electrode layer 30. Next, while the temperature of the substrate 10 and the atmosphere was kept at the room temperature, the substance such as Au was vapor-deposited in the thickness of about 200 nm as the lead electrode layer 50. (These samples correspond to the samples No. Al through A15.) Also, samples No. B7 through B8 were prepared as follows. while keeping the temperature of the substrate 10 and the atmosphere at the room temperature, a IVA-group metal or alloy was vapor-deposited as the contact electrode layer 30. Then, the resultant was subjected to an annealing treatment to heat it at a temperature of lower than 300 °C in vacuum of below 666,6 Pa (5 Torr) for thirty minutes. Next, the temperature of the atmosphere was reduced down to the room temperature and then the substance such as Au was vapor-deposited in the thickness of about 200 nm as the lead electrode layer 50. (These samples correspond to the samples No. A16 through A19.) Further, samples No. B9 and B10 were prepared as follows. while keeping the temperature of the substrate 10 and the atmosphere at the room temperature, a IVA-group metal was vapor-deposited as the contact electrode layer 30. Then, the resultant was subjected to an annealing treatment to heat it at a temperature of lower than 300 °C in vacuum of below 666,6 Pa (5 Torr) for thirty minutes. Next, the contact electrode layer 30 was washed with an acid solution such as hydrofluoric acid or hydrofluoric acid/nitric acid to remove the metal or alloy remaining without forming the carbide. Subsequently, the temperature of the atmosphere was reduced down to the room temperature and then the substance such as Au was vapor-deposited in the thickness of about 200 nm as the lead electrode layer 50. (These samples correspond to the samples No. A23 through A25.) The thus formed samples No. B1 through B10 were subjected to a heat treatment at a temperature of 500 °C in vacuum of below 133,32·10 -5 Pa (10 -5 Torr) for thirty minutes. The resistivity of the lead wire resistance was measured by the four-probe method before and after the heat treatment to evaluate the heat resistance, based on a change in the resistivity. The results of the measurement demonstrated that samples No. B1 through B8 before the heat treatment had the lead wire resistance approximately equal to the resistance value of Au normally formed in bulk, but the samples after the heat treatment had increased resistance. Change rates of the resistance were considerably large, over ten times. Also, the surface of the lead electrode layer 50 showed completely different colors before and after the heat treatment. Analysis of composition was conducted for the surface of the lead electrode layer 50, which showed that the metal or alloy forming the contact electrode layer 30 precipitated and no Au remained. It is thus understood that satisfactory heat resistance was not achieved. On the other hand, samples No. B9 and B10 before the heat treatment showed the lead wire resistance approximately equal to the resistance value of Au normally formed in bulk, but the samples after the heat treatment increased the resistance. Change rates of the resistance were large, i.e., several times. It is thus understood that satisfactory heat resistance was not achieved. In another experiment, the samples No. B1 through B10 were subjected to a heat treatment at a temperature of 500 °C in vacuum of below 133,32·10 -5 Pa (10 -5 Torr) for thirty minutes. The contact resistance was measured by the TLM method before and after this heat treatment to evaluate the ohmic property to the p-type diamond layer 20, based on a change in the contact resistance. The results of the measurement demonstrated that the samples No. B9 and B10 showed large values of the contact resistance before and after the heat treatment. For these electrodes, element analysis was conducted in the direction of the thickness. The results of the analysis showed that little element forming the contact electrode layer 30 was detected near the interface to the p-type diamond layer 20. This indicates that because the temperature of the annealing treatment after the formation of the contact electrode layer 30 was low, i.e., below 300 °C, the metal or alloy forming the contact electrode layer 30 did not form the carbide thereof well near the interface to the p-type diamond layer 20 whereby the metal or alloy forming the contact electrode layer 30 was removed when washed with the acid solution. It is thus understood that satisfactory ohmic property was not achieved. On the other hand, the samples No. B1 through B8 showed small values of the contact resistance before and after the heat treatment similarly. It is thus understood that satisfactory ohmic property was achieved. As detailed above, the ohmic electrode of the present invention is so arranged that the contact electrode layer is made of the carbide of at least one metal selected from the metal group consisting of Ti, Zr, and Hf, or the carbide of an alloy containing at least one metal selected from the said metal group and the contact electrode layer is formed in ohmic contact with the p-type diamond semiconductor layer. In the arrangement, the carbide of the metal or alloy forming the contact electrode layer is stabler in energy than the metal or alloy itself, so that it is very unlikely to diffuse. Then, little metal or alloy precipitates on the surface of the lead electrode layer, thus lowering the lead wire resistance.

Claims: An ohmic electrode comprising: a contact electrode layer (30) formed on a p-type diamond semiconductor layer (20) formed on a substrate (10) so as to have low contact resistance and high heat resistance and to be in ohmic contact with said p-type diamond semiconductor layer (20); and a lead electrode layer (50) directly formed on said contact electrode layer (30) and made of Au so as to have low lead wire resistance and high heat resistance; characterized in that: said whole contact electrode layer (30) is made of either a carbide of at least one metal selected from a metal group consisting of Ti, Zr and Hf, or carbide of an alloy including at least one metal selected from said metal group. A method for forming an ohmic electrode according to claim 1, comprising: a first step of vapor-depositing either at least one metal selected from a metal group comprising Ti, Zr, and Hf or a carbide thereof, or an alloy containing at least one metal selected from said metal group or a carbide thereof on a p-type diamond semiconductor layer (20) formed on a substrate (10) to form a contact electrode layer (30) being in ohmic contact with said p-type diamond semiconductor layer (20) and having low contact resistance and high heat resistance; and a second step of forming by vapor deposition a lead electrode layer (50) made of Au on said contact electrode layer (30) formed in the first step; characterized in that: said second step is sequentially performed after said first step, and that said first step comprises a heat treatment to heat said contact electrode layer (30) at a temperature of not lower than 300°C. A method according to claim 2, wherein said heat treatment is to heat said substrate (10) at a temperature of not lower than 300°C in forming said contact electrode layer (30) on said p-type diamond semiconductor layer (20) by vapor deposition. A method according to claim 2, wherein said heat treatment is to heat said contact electrode layer (30) in an atmosphere having a temperature of not lower than 300°C after the formation of said contact electrode layer (30) on said p-type diamond semiconductor layer (20) by vapor deposition.

Inventors: NISHIBAYASHI YOSHIKI, SHIKATA SHIN-ICHI, TODA NAOHIRO, TOMIKAWA TADASHI, NISHIBAYASHI,YOSHIKI, SHIKATA, SHIN-ICHI, TODA, NAOHIRO, TOMIKAWA, TADASHI, NISHIBAYASHI,YOSHIKI, C/O ITAMI WORKS OF SUMITOMO, SHIKATA, SHIN-ICHI, C/O ITAMI WORKS OF SUMITOMO, TODA, NAOHIRO, C/O ITAMI WORKS OF SUMITOMO, TOMIKAWA, TADASHI, C/O ITAMI WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1994
Main classification: H01L29
IPC codes: H01L29, H01L33, H01L21